Structural and magnetic properties of Zn1−xSbxCr2−x/3Se4 (x=0.11, 0.16 and 0.20) single crystals

Single crystals of Zn_1−xSb_xCr_2−x/3Se₄ based on the ZnCr₂Se₄ spinel, which is known to exhibit interesting magnetic and electronic transport properties, have been prepared by solid state reaction from the appropriate selenides. Three compounds of different Sb content (x=0.11, 0.16, and 0.20) were studied by X-ray diffraction, X-ray photoelectron scattering technique and macroscopic magnetic measurements with the aim to determine (i) stability of the cubic symmetry and (ii) influence of the Sb admixture on the magnetic properties. The results show that the Sb³⁺ and Zn²⁺ ions share the tetrahedral sites in the spinel structure, while the Cr³⁺ions carrying magnetic moments, are located in the octahedral sites. The X-ray photoelectron spectroscopy (XPS) data indicate that in this series of compounds the chromium ions have a 3d³ electronic configuration. The three samples studied order antiferromagnetically at low temperatures, with the magnetic characteristics being hardly altered with respect to those reported for the parent ZnCr₂Se₄ compound.     

Synthesis, structure and physical properties of the new Zintl phases Eu11Zn6Sb12 and Eu11Cd6Sb12

Reported are the syntheses, crystal structure determinations from single-crystal X-ray diffraction, and magnetic properties of two new ternary compounds, Eu₁₁Cd₆Sb₁₂ and Eu₁₁Zn₆Sb₁₂. Both crystallize with the complex Sr₁₁Cd₆Sb₁₂ structure type—monoclinic, space group C2/m (no. 12), Z=2, with unit cell parameters a=31.979(4) Å, b=4.5981(5) Å, c=12.3499(14) Å, β=109.675(1)° for Eu₁₁Zn₆Sb₁₂, and a=32.507(2) Å, b=4.7294(3) Å, c=12.4158(8) Å, β=109.972(1)° for Eu₁₁Cd₆Sb₁₂. Their crystal structures are best described as made up of polyanionic and ribbons of corner-shared ZnSb₄ and CdSb₄ tetrahedra and Eu²⁺ cations. A notable characteristic of these structures is the presence of Sb-Sb interactions, which exist between two tetrahedra from adjacent layers, giving rise to unique channels. Detailed structure analyses shows that similar bonding arrangements are seen in much simpler structure types, such as Ca₃AlAs₃ and Ca₅Ga₂As₆ and the structure can be rationalized as their intergrowth. Temperature-dependent magnetization measurements indicate that Eu₁₁Cd₆Sb₁₂ orders anti-ferromagnetically below 7.5 K, while Eu₁₁Zn₆Sb₁₂ does not order down to 5 K. Resistivity measurements confirm that Eu₁₁Cd₆Sb₁₂ is poorly metallic, as expected for a Zintl phase.     

Structure and physical properties of nonstoichiometric rare-earth cadmium antimonides, RECd1−xSb2 (RE=La, Ce, Pr, Nd, Sm)

The ternary rare-earth cadmium antimonides RECd_1−xSb₂ (RE=La, Ce, Pr, Nd, Sm) were prepared by reaction of the elements at 1000 °C. The presence of Cd defects, previously found for LaCd_0.700(5)Sb₂ and CeCd_0.660(4)Sb₂, has been confirmed by single-crystal X-ray diffraction studies for the isotypic compounds PrCd_0.665(3)Sb₂, ), NdCd_0.659(3)Sb₂, ), and SmCd_0.648(3)Sb₂, ). These compounds adopt the HfCuSi₂-type structure (Pearson symbol tP8, space group P4/nmm, Z=2). The electrical and magnetic properties of samples with nominal composition RECd_0.7Sb₂ were investigated. All exhibit metallic behaviour, but CeCd_0.7Sb₂ undergoes an abrupt drop in its electrical resistivity below 3 K. LaCd_0.7Sb₂ exhibits temperature-independent Pauli paramagnetism and SmCd_0.7Sb₂ displays van Vleck paramagnetism. The remaining compounds obey the modified Curie-Weiss law at high temperatures. CeCd_0.7Sb₂ undergoes ferromagnetic ordering below 3 K, reaching a saturation magnetization of ∼1.0 μ_B, whereas PrCd_0.7Sb₂ and NdCd_0.7Sb₂ remain paramagnetic down to 2 K.     

Influence of the structure on electric and magnetic properties of La0.8Na0.2Mn1−xCoxO3 perovskites

The influence of the cobalt substitution for manganese ions in the mixed valence perovskites La_0.8Na_0.2Mn_1−xCo_xO₃ (0?x?0.2) was investigated by X-ray, electric transport and magnetic measurements. The study carried out on sintered polycrystalline samples revealed the rhombohedral () structure and the insulator-metal transition connected with a ferromagnetic arrangement in the whole concentration range. Increasing concentration of cobalt ions leads to a gradual decrease of PM-FM and I-M transition temperatures. An influence of the cobalt ions on the observed behavior is attributed to charge compensation Mn³⁺→Mn⁴⁺ leading to the formation of stable couples Mn⁴⁺-Co²⁺. Therefore the double-exchange interactions Mn³⁺-O²⁻-Mn⁴⁺ partly vanish and they are replaced by positive superexchange interactions Mn⁴⁺-O²⁻-Co²⁺, but of a semiconducting character.     

Synthesis and magnetic properties of the quasi-one-dimensional compound Ca0.83(Cu1−xCox)O2

A new solid solution of the quasi-one-dimensional composite crystal, , has been synthesized under of O₂ at 830°C. The non-doped compound Ca_0.83CuO₂ consists of two interpenetrating monoclinic subsystems of the [Ca] atoms and the edge-shared square planar [CuO₂] chains. Upon increasing x, both the subsystems undergo a phase change from monoclinic to orthorhombic (M-O). The M-O change occurs at x∼0.04 for the [(Cu,Co)O₂] subsystem, while such a change occurs at x∼0.17 for the [Ca] subsystem. Magnetic susceptibility measurements show an evolution from a short-range ordered state near x=0 to a long-range antiferromagnetic state for the samples with x?0.15. The effective magnetic moment μ_eff is found to increase with increasing x from for x=0.10 to for x=0.30, suggesting that the solid solution can be regarded as Ca_0.83[Cu_0.66²⁺Cu_0.34−x³⁺Co_x³⁺]O₂, in which a mixed state of Cu²⁺(S=1/2), Cu³⁺(S=0) and high-spin Co³⁺(S=2) ions is realized.     

Defect structure of Sb2−xFexTe3 single crystals

Single crystals of Sb_2−xFe_xTe₃ (c_Fe=0-9.5×10¹⁹ cm⁻³) were prepared by Bridgman method. The interpretation of the reflection spectra in plasma resonance region indicates that Fe increases the concentration of holes (acceptor) and each Fe atom incorporated in Sb₂Te₃ structure liberates 0.4-0.5 hole. Observed effect is elucidated by means of point defect model. According to the model Fe atoms enter the structure and form uncharged substitutional defects . Since this defect cannot affect the free-carrier concentration directly, we assume an interaction of the entering Fe-atoms with natives defects leading to a rise in the concentration of antisite defects , to a decrease of concentration, and to an increase in the concentration of holes.     

Anion substitution effects on structure and magnetism in the chromium chalcogenide Cr5Te8—Part I: Cluster glass behavior in trigonal Cr(1+x)Q2 with basic cell (Q=Te, Se; Te:Se=7:1)

The effect of substitution of the anion Te by Se in non-stoichiometric Cr₅Te₈ has been investigated with respect to its crystal structure, magnetic properties, and electronic structure. The compounds Cr_(1+x)Q₂ (Q=Te, Se; Te:Se=7:1; (1+x)=1.234(6), 1.264(6), 1.300(7)) were synthesized at elevated temperatures followed by quenching the samples to room temperature. The crystal structures have been refined with X-ray powder diffraction data with the Rietveld method in the trigonal space group with lattice parameters a=3.8651(1)-3.8831(1) Å and c=5.9917(2)-6.0528(2) Å. The structure is related to the NiAs structure with full and deficient metal layers stacking alternatively along the c-axis. The irreversibility in the field-cooled/zero-field-cooled magnetization suggests that the substitution effects of one Te by one Se is strong enough to cause cluster-glass behavior, from ferromagnetic Cr₅Te₈ to cluster-glass Cr_(1+x)Q₂. Non-saturation magnetizations at 5.5 T and the magnetic relaxation results further support the existence of cluster-glass behavior. Accompanying SPR-KKR (spin-polarized relativistic Korringa-Kohn-Rostoker) band structure calculations strongly support the observation that the Cr(1) sites are preferentially occupied by Cr atoms and predict that these compounds are metallic. Results for the spin-resolved DOS and magnetic moments on each crystallographic sites are presented.     

A TEM investigation of the (Bi1−xSrx)FeO3−x/2, 0.2≤x≤0.67, solid solution and a suggested superspace structural description thereof

A careful transmission electron microscopy (TEM) investigation of an incommensurately modulated member of the (Bi_1−xSr_x)Fe³⁺O_3−x/2□_x/2, 0.2≤x≤0.67, solid solution has been carried out. High resolution (HR) TEM imaging is used to show the presence of at least 6-fold twinning on a rather fine (5 nm) scale. The (3+1)-d superspace group symmetry is suggested to be or one of the non-centrosymmetric sub-groups thereof, namely , , and . A superspace construction is then used to propose the nature of the local compositional ordering and, hence, of the oxygen-deficient slab that intergrows with the perovskite slab to produce the observed solid solution phase. The proposed compositional superspace atomic surfaces can be used to produce model structures at any composition within the solid solution range.     

Independent structural and valence state transitions in the cation-ordered double perovskites Ba2−xSrxTbIrO6

Synchrotron X-ray and neutron powder diffraction were used to investigate the formation, structure and bonding in the double perovskite Ba_2−xSr_xTbIrO₆ solid solutions. The results showed that these oxides all exhibit ordering of the Tb and Ir cations in a double perovskite-type structure. Three distinct structural types differing in symmetry and/or valence states were formed depending on the precise Ba:Sr ratio on the perovskite A site; x?0.3 cubic () with Tb⁴⁺ and Ir⁴⁺; 0.4?x?1.0 cubic () with Tb³⁺ and Ir⁵⁺ and x?1.2 monoclinic (P2₁/n) with Tb³⁺ and Ir⁵⁺. The transitions between these appear to be first order in nature.     

Ba5Ti12Sb19+x, a polar intermetallic compound with a stuffed γ-brass structure

The polar intermetallic compound Ba₅Ti₁₂Sb_19+x (x0.2) has been synthesized by reaction of the elements. Single-crystal X-ray diffraction analysis revealed that it adopts a new structure type (Ba₅Ti₁₂Sb_19.102(6), space group , Z=2, a=12.4223(11) Å, V=1916.9(3) Å³). The set of Ba and Sb sites corresponds to the structure of Cu₉Al₄, a γ-brass type with a primitive cell. A complex three-dimensional framework of Ti atoms, in the form of linked planar Ti₉ clusters, is stuffed within the γ-brass-type Ba–Sb substructure. Notwithstanding its relationship to the γ-brass structure, the compound does not appear to conform to the Hume–Rothery electron concentration rules. Band structure calculations on an idealized Ba₅Ti₁₂Sb₁₉ model suggest that the availability of bonding states above the Fermi level is responsible for the partial occupation, but only to a limited degree, of an additional Sb site within the structure. Magnetic measurements indicated Pauli paramagnetic behaviour.     

Magnetic and vibrational properties and crystal structure of Sr9.2Co1.3(PO4)7 with disordered arrangements of some strontium, cobalt, and phosphate ions

Solid solutions of Sr_9+xCo_1.5−x(PO₄)₇ were found in the compositional range of 0.05?x?0.30. The structure of Sr_9.2Co_1.3(PO₄)₇ (x=0.2) was determined from single crystal X-ray diffraction (space group (No. 166); Z=3; and ; ; ; ) and refined to R₁=0.0343 and wR₂=0.0633 for 586 reflections with I>2σ(I). Sr_9.2Co_1.3(PO₄)₇ is structurally related to β-Ca₃(PO₄)₂ and Sr₃(PO₄)₂ and has disordered arrangements of some Sr²⁺, Co²⁺, and PO₄³⁻ ions. Sr²⁺ ions at a 9e site are statistically disordered among four positions near the center of symmetry. Co²⁺ and Sr²⁺ ions are split along the c-axis to occupy a 6c site that is 75% vacant. The P1O₄ tetrahedra are orientationally disordered. Sr²⁺ ions at an 8-fold coordinated 18h site, Co²⁺ ions at an octahedral 3a site, and the P2O₄ tetrahedra are ordered in the structure of Sr_9.2Co_1.3(PO₄)₇. Features of Raman spectra are discussed in relation to the crystallographic structure of Sr_9.2Co_1.3(PO₄)₇ and in comparison with Raman spectra of β-Ca₃(PO₄)₂-type and Sr₃(PO₄)₂-type compounds. Sr_9.2Co_1.3(PO₄)₇ is paramagnetic between 2 and 300 K with an effective magnetic moment of 4.98μ_B per Co²⁺ ion.     

The Pr2Se3-PrSe2 system: Studies of the phase relationships and the modulated crystal structure of PrSe1.85

Thermochemical investigations have been carried out to elucidate the phase relationships in the system Pr₂Se₃-PrSe₂ and to construct the p_Se-T-x diagram. The result of the tensimetric and thermogravimetric studies showed the thermal decomposition of PrSe₂ to be a step-wise process due to the formation of a sequence of discrete intermediate phases with the compositions PrSe_1.9, PrSe_1.85, PrSe_1.8, and Pr₂Se₃. PrSe_1.85 and PrSe_1.8 have been previously considered as selenium-deficient non-stoichiometric phases. Conclusions derived from the thermodynamic study were verified by X-ray diffraction and electron microscopy studies on single crystals of PrSe_1.85 which were obtained from reactions of praseodymium and selenium in stoichiometric amounts in a KCl flux at 1070 K. A two-dimensional modulation could be observed for PrSe_1.85 in X-ray and high-resolution transmission electron microscopy. The structure of PrSe_1.85 was solved and refined in superspace group P4/n()(β-α)00 with lattice parameters of a=4.137(1) Å, c=8.398(2) Å of the basic unit cell and α=β=0.293(1). The origin of the modulation can be attributed to a site occupancy wave and a charge density wave in its planar selenium layer. The experimentally determined magnetic moments indicate Pr³⁺ for the polyselenides PrSe₂, PrSe_1.9 and PrSe_1.85.     

Correlation between structure and magnetic properties of Cd-substituted La0.7(Ca0.3−xCdx)MnO3 CMR manganites

Structural, electrical and magnetic properties of Cd-doped La_0.7(Ca_0.3−xCd_x)MnO₃ (0?x?0.3) manganites are presented. All compositions were indexed in the orthorhombic (Pnma) space group, except the Cd_0.3 sample, indexed as a combination of trigonal and orthorhombic (Pnma) space groups. Substitution of Ca by Cd has a strong influence on the magnetic and magnetoresistive properties of these compounds, continuously decreasing both the magnetic moment and the Curie temperature (from 3.5 μ_B and 270 K for the x=0 composition to 1.59 μ_B and 90 K for the fully doped x=0.3 one). Samples corresponding to x=0 and 0.1 show a semiconductor-metal transition at temperatures close to the Curie ones. The measured magnetoresistance change is about 49% at 270 K and 95% at 165 K for those samples, respectively. However, the x=0.2 and 0.3 compositions show insulating behaviour in the whole temperature range studied, with values of the magnetoresistance about 85% at 105 K and 74% at 90 K, respectively. The observed weakening of the double-exchange mechanism as the Cd doping level in these samples increases is discussed in terms of structural properties, cationic disorder and Mn³⁺/Mn⁴⁺content ratio.     

Substitution of Co in YBa2Fe3O8

The accommodation of Co in the oxygen-saturated solid-solution phase YBa₂(Fe_1−zCo_z)₃O_8+w has been investigated by powder X-ray and neutron diffraction techniques, as well as by Mössbauer spectroscopy. Of the nominal composition range 0.00?z?1.00 tested, the solid-solution limit under syntheses at 950°C in is z=0.47(5). No symmetry change in the nuclear and magnetic structures is seen as a consequence of the Co substitution, and the Co atoms are distributed evenly over the two sites that are square-pyramidally and octahedrally coordinated for w=0. The oxygen-saturated samples maintain their oxygen content roughly constant throughout the homogeneity range, showing that Co³⁺ replaces Fe³⁺. Despite the nearly constant value of w, Mössbauer spectroscopy shows that the amount of tetravalent Fe slightly increases with increasing z, and this allows Co to adopt valence close to 3.00 to a good approximation. The magnitude of the antiferromagnetic moment (located in the a,b plane) decreases with z in accordance with the high-spin states of the majority Fe³⁺ and Co³⁺ ions. Bond-valence analyses are performed to illustrate how the structural network becomes increasingly frustrated as a result of the substitution of Fe³⁺ by the smaller Co³⁺ ion. A contrast is pointed out with the substitution of cobalt in YBa₂Cu₃O₇ where it is a larger Co²⁺ ion that replaces smaller Cu²⁺.     

The crystal structure of the new ternary antimonide Dy3Cu20+xSb11−x (x≈2)

New ternary antimonide Dy₃Cu_20+xSb_11−x (x≈2) was synthesized and its crystal structure was determined by direct methods from X-ray powder diffraction data (diffractometer DRON-3M, CuKα-radiation, R_I=6.99%,R_p=12.27%,R_wp=11.55%). The compound crystallizes with the own cubic structure type: space group , Pearson code cF272, . The structure of the Dy₃Cu₂₀Sb_11−x (x≈2) can be obtained from the structure type BaHg₁₁ by doubling of the lattice parameter and subtraction of 16 atoms. The studied structure was compared with the structures of known compounds, which crystallize in the same space group with similar cell parameters.     

REAuAl4Ge2 and REAuAl4(AuxGe1−x)2 (RE=rare earth element): Quaternary intermetallics grown in liquid aluminum

The two families of intermetallic phases REAuAl₄Ge₂ (1) (RE=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm and Yb) and REAuAl₄(Au_xGe_1−x)₂ (2) (x=0.4) (RE=Ce and Eu) were obtained by the reactive combination of RE, Au and Ge in liquid aluminum. The structure of (1) adopts the space group R-3m (CeAuAl₄Ge₂, , ; NdAuAl₄Ge₂, , ; GdAuAl₄Ge₂, , ; ErAuAl₄Ge₂, , ). The structure of (2) adopts the tetragonal space group P4/mmm with lattice parameters: , for EuAuAl₄(Au_xGe_1−x)₂ (x=0.4). Both structure types present slabs of “AuAl₄Ge₂” or “AuAl₄(Au_xGe_1−x)₂” stacking along the c-axis with layers of RE atoms in between. Magnetic susceptibility measurements indicate that the RE atoms (except for Ce and Eu) possess magnetic moments consistent with +3 species. The Ce atoms in CeAuAl₄Ge₂ and CeAuAl₄(Au_xGe_1−x)₂ (x=0.4) appear to be in a mixed +3/+4 valence state; DyAuAl₄Ge₂ undergoes an antiferromagnetic transition at 11 K and below this temperature exhibits metamagnetic behavior. The Eu atoms in EuAuAl₄(Au_xGe_1−x)₂ (x=0.4) appear to be in a 2+ oxidation state.     

High-pressure synthesis of solid solutions between trigonal LiNiO2 and monoclinic Li[Li1/3Ni2/3]O2

High-pressure synthesis in an oxygen-rich atmosphere yields solid solutions between LiNiO₂ and Li₂NiO₃ over the whole concentration range. Structural characterization of the high-pressure oxides was performed using powder XRD, SEM analysis, IR spectroscopy, EPR spectroscopy at 9.23 and 115 GHz and magnetic susceptibility measurements. The crystal structure of Li[Li_xNi_1−x]O₂ ,, changes from trigonal R-3m to monoclinic C2/m at Li-to-Ni ratio of 2 (or ). The incorporation of Li into NiO₂-layers causes a decrease in the mean Li-O and Ni_1-xLi_x-O bond distance. Li and Ni ions in the mixed Ni_1-xLi_xO₂-layers display a tendency to order at a short length scale in such a way that mimics the Li_1/3Ni_2/3-arrangment of the end Li[Li_1/3Ni_2/3]O₂ composition. The charge distribution in these oxides proceeds via Ni³⁺ and Ni⁴⁺ ions.     

The crystal structure of CuSb2O3Br: Slabs from cubic Sb2O3 interspersed between puckered hexagonal CuBr-type layers

The new compound CuSb₂O₃Br crystallize in the monoclinic space group Cc. The unit cell parameters are , , , β=90°, Z=16. The crystal structure is solved from single crystal data, R=0.0490. The compound show a layered structure with slabs from cubic Sb₂O₃ interspersed in between puckered layers of CuBr. The Sb(III) atoms have tetrahedral [SbO₃E] coordination where E is the 5s² lone pair, these units build up Sb₄O₄E₆ cages. The CuBr layers resemble those in hexagonal CuBr but the Cu(I) ions have actually tetrahedral [CuBr₃O] coordination. The Cu-O bonds link the Sb₄O₆ cages with the CuBr layers.     

Topotactic Oxidation of the Quadruple-Rutile-Type Chain Structure Na0.875Fe0.875Ti1.125O4

Sodium removal from Na_0.875Fe_0.875Ti_1.125O₄by means of oxidizing agents leads to the formation of the defect solid solution Na_0.875−δFe_0.875−δTi_1.125O₄(0≤δ≤0.44). A systematic increase in theparameter of the orthorhombic unit cell is observed as the sodium content is reduced, while theparameter decreases only slightly. The cell volume remains almost constant as a consequence of the irregular change in theparameter and the different behavior of both theandparameters. Structural changes have been followed by Rietveld powder diffraction analysis. Room- and low-temperature Mössbauer studies confirm the increase in the Fe⁴⁺state with decreased sodium content in the less extracted samples withδ=0.10 and 0.15, whereas a maximum Fe⁴⁺content (about 21%) is reached in a sample withδ=0.25. Surprisingly, the amount of Fe⁴⁺does not increase with further sodium extraction. This is thought to be related to the greater oxidative power of the more oxidized materials, which facilitates a subsequent partial backreduction of Fe⁴⁺formed during the oxidation procedure. Results of both Mössbauer spectroscopy and structure refinements indicate that the Fe⁴⁺cations randomly occupy both octahedral metal positions,M(1) andM(2), of each quadruple rutile unit.